This is a question which I’ve often asked, but it wasn’t until I was editing Her Life Collected that I started to get some answers.  Note that these answers didn’t come when I was writing the poems, mind you, but when I was editing. I found that fascinating.  Over the years as I have had more and more work published, and more and more people wonder about my work, I have always said that the work itself dictates the form.  When an idea comes along, it is clear whether it is a poem, a novel or a play, I have often said.  But I must admit, that is not altogether true.  Those who read my poems (and of course, I hope many of you will) will find that many of them are narrative.  They often describe moments in time, which often includes social interactions and dialogue. And so the question arises, how do I choose my line breaks, and as it turns out, it is this very choice which dictates the difference between poetry and prose/prose poetry.  My editor/publisher Adele Ward and I had a lengthy discussion about this issue and she had some fascinating insights:
There has to be a reason for each line break in a poem, and if there isn’t a reason, then it’s a short piece of poetic prose (a prose poem). No reason not to present it in that way. Some lines just fall with the flatness of prose in a poem. You can hear it. It’s not that there’s anything wrong with prose, but the ear can hear a line of prose in a poem, especially when you have a long line that has gone into prose. This might happen in a draft, especially if you write in both forms, and at parts where you’re wanting to get the poem down while it’s in your mind but can’t quite manage yet. Afterwards you would want to take those parts out and use them like notes so you can complete the poem. Sometimes you may want to write poems that alternate prose sections with poetry.
Line breaks are one of the trickiest considerations in poetry. Andrew Motion made me really think about mine all the time. A line break serves a few purposes. One can be that the line breaks work with the rhythm when you read it out loud. If they don’t then it’s prose randomly cut into a poetic stanza. Another reason is to put that little emphasis on the words at line end and particularly at line beginning. One wonderful poet (I know) describes the movement from the end of a line of poetry to the next line as the feeling of stepping off the shore on to a little boat. This stresses the two words, gives a rhythm, and also can work in a lovely neat way if there’s also a little movement in the meaning that’s being reflected. Like ‘the leaves/falling from the trees’. If none of these are your reasons and you have a piece that’s prose-like but you want it to be shaped like the rest of the poem, then each line can be a step forward in the meaning. Actually if the line breaks are just arbitrary to make something look like a poem, and if the rhythm reads like prose, then it would be better presented as a prose poem. There’s absolutely no reason for the line breaks. 

As it turns out, I have several prose poems in my book and I didn’t even realize that was what they were when I was writing them. Here’s one which actually relies mostly on dialogue:

Urban Doorstep

Lock the top. Lock the bottom. The alarm sets with two long
beeps, one for each step to the street.


Excuse me. Are you happy with your window cleaner?

My neighbour is watering her window box. A short arm
extended over the wrought iron railing. It’s the beginning of
spring.

I noticed him the other day. He’s good?
‘Yes, and reliable. But expensive.’
It’s worth it, though – both inside and out. Especially at this time
of year.
‘Yes, a little sun.’
It’s been a long winter.
‘Yes, it has.’
Yes, it has. She picks a paint flake off the rail. Two grey
strands steal away across her brow, escapees from the styled
mass.
‘I meant to introduce myself. I’ve been a horrible
neighbour.’
Oh, it’s quite all right. We’re all so busy. The time does go so fast.

Something in her face keeps me there. The normal, sunken
space beneath her cheek; the puffiness round her eyes. My
right hip settles in its socket. I place my bag at my feet and
lean against the rail.

‘Settling into a new home – it’s so much work, I know.’
I do love the street. But it has been a difficult year. It’s my
husband. Well, actually, it’s us –
‘Oh, I’m so sorry.’
Thirty years of marriage. I took it very badly.
‘How could you not?’
I’m better now. I’m going to the theatre tonight.
‘Yes, you must get out. And you must come for a drink.’
I’d like that. And you must come to me. Then silence.
‘But the window washer’s number. I’ll put it through your
door.’
Yes, please do. It will be good to have sun in these rooms.